Not Applicable
Not Applicable
1. Field of the Invention
The present invention pertains generally to methods for driving solenoids and more particularly to a method of estimating heat accumulation within solenoids and providing compensation thereof.
2. Description of the Background Art
A wide array of electromechanical systems utilize solenoids for converting electrical signals into a mechanical counterpart. One such system is that of an electronic player piano utilizing electrical solenoids for driving the keys of the piano. A solenoid is an electromechanical device having a conductive coil which is retained proximal to a mechanical assembly having one or more magnetic components capable of movement in response to changes in the magnetic field of the conductive coil. Solenoids can generate various forms of motion, such as radial, or linear. The use of a solenoid may be generally separated into one of two general classes.
The first class of use is xe2x80x9cnon-linearxe2x80x9d, in which the solenoid is typically retained in one of a number of fixed states, typically two. When utilized non-linearly, the purpose of the solenoid is to readily reach the fixed state, therefore the solenoid is generally driven toward one of the fixed states under a predetermined set of fixed drive conditions, or set drive voltage. For example a drive transistor is switched on to source or sink current through the coil of the solenoid so that the translation mechanism, such as a cylindrical plunger, quickly moves to an active state. The drive transistor is then typically maintained in the given active state until the translation mechanism is to be moved back toward the non-active state, wherein the transistor is switched off to stop the flow of coil current. Alternatively, solenoids may be driven by other means such as by a push-pull driver circuit, and so forth.
The second class of use may be termed xe2x80x9clinearxe2x80x9d, wherein the solenoid is linearly driven to achieve intermediate positions or desired characteristics of motion, such as a given speed, or force, at a particular point in time. When driving a solenoid in linear mode, the electromagnetic and mechanical characteristics of the circuit, solenoid, and the coupled mechanical device must be taken into account. Solenoids driven in a xe2x80x9clinearxe2x80x9d manner may be utilized in a number of applications wherein the position, velocity, acceleration, or force of the solenoid output is to be linearly controlled. One application for which linear solenoid drive is preferable is within electronic player-piano mechanisms. The linear solenoid driving refers to the response of the solenoid and is not to be confused with the input signal applied to the solenoid. It will be appreciated that the output of a solenoid may be linearly controlled by the application of either analog or digital signals, for example in a similar manner that analog audio may be generated from passing analog signals through a xe2x80x9cClass Axe2x80x9d amplifier, or by using digital signals and a xe2x80x9cClass Dxe2x80x9d amplifier. A number of applications require that the coil be driven in an analog manner to achieve the desired output characteristics for the system. One such application is that of player pianos utilizing electronic player mechanisms which incorporate electrical solenoids to control activation of the keys, such as the eighty eight (88) keys contained on a conventional piano keyboard, in response to composition data.
The majority of modern player pianos utilize electrical solenoids for driving the keys of the piano. The signals applied to the solenoids allow for the control of key speed and force. Changes in solenoid movement are accomplished by altering the drive voltages being applied to the coil of the solenoid. The amount of current flow through the coil is determined by the drive circuits and the resistance of the coil. Heat is generated as a by-product of the current flowing through the resistance of the coil and as a result the temperature of the solenoid can increase significantly under recurrent or extended operation. In response to increases in coil temperature, the amount of coil resistance increases and the drive characteristics of the solenoid are thereby altered. One of the negative aspects of the solenoid temperature increase is the resultant decrease in the coil drive current associated with any given drive voltage. As a result, the velocity, acceleration, and force of the solenoid mechanism diminishes as the solenoid heats up. Within player pianos the force roll-off is exhibited as an attenuation of audio volume on keys which have been played repeatedly, thus skewing the reproduction of accurate sound. FIG. 1 shows audio volume roll-off in response to coil temperature increases as a key is repeatedly activated over a period of time. It can be seen from the graph that the audio volume drops approximately thirteen decibels (13 db) over a thirteen minute test interval. It will be appreciated that a drop of six decibels (6 db) equates to an amplitude drop of approximately one half. Continued energizing of the coil at a sufficient current level results in the coil temperature reaching an equilibrium temperature in which the rate of heat dissipation equals the amount of heat generation in the coil.
It has been recognized, therefore, that accurately driving a solenoid requires taking the solenoid coil temperature into account. One proposed method of compensating for solenoid temperature changes requires the incorporation of a temperature sensor within each solenoid. It will be appreciated that the addition of eighty eight (88) temperature sensor and conditioning circuitry to a player piano can substantially increase both the cost and complexity of the player mechanism. The temperature of the solenoid is periodically measured and the electrical output of the solenoid drive circuit is adjusted accordingly to reduce sound changes in response to solenoid temperature. Furthermore, the temperatures measured by these individual temperature sensors are subject to a delay error as they are typically mounted on the exterior of the coil and themselves have a thermal mass, wherein the temperature registered by the temperature sensor can be out of phase with the temperature profile of the solenoid at its core for which the compensation is desired.
Therefore, a need exists for a method of determining solenoid temperature without adding temperature sensors to each solenoid. The present invention satisfies those needs, as well as others, and overcomes the deficiencies of previously developed solutions.
The present invention provides a method of estimating the heat accumulated within a solenoid without the necessity of adding a temperature sensor for each solenoid. The described method eschews the cost and reliability issues inherent in adding temperature sensors and allows for compensation of solenoid drive power in response to solenoid heat accumulation.
The method described for the present invention recognizes that, given a reasonably constant ambient temperature, the temperature of the solenoid is typically a function of its recent activation history. The heat accumulated by the solenoid is estimated by registering the power absorbed by the solenoid during periods of activation, while subtracting an amount of heat proportional to the heat dissipation. It will be appreciated that heat is always being dissipated by a solenoid that has accumulated heat, and that the temperature of an inactive solenoid will move toward the ambient temperature. The temperature, however, of solenoids which are subject to sufficient levels of activation will attain a thermal equilibrium condition wherein continued activations will not continue to raise the accumulated heat of the solenoid.
The temperature of the solenoid therefore increases during activation as a function of applied current, the thermal mass, and the thermal resistance of the solenoid coil. Upon becoming inactive, the solenoid temperature drops toward the ambient temperature as a function of the extent of temperature elevation, the thermal mass, and the thermal resistance of the solenoid coil. The method of the present invention tracks the power applied to each solenoid and the cooling effects of thermal heat dissipation. In applications designed to minimize motion and force fluctuations, or applications subject to a narrow ambient temperature range, an actual value for solenoid temperature need not be maintained and there is consequently no need to measure the actual ambient temperature. The aforementioned player piano application is one such example of a system utilizing solenoids that is well suited to non-absolute value temperature estimation. In the case of a player piano, the ambient temperature surrounding the piano is held in a narrow range of temperature while it is the relative changes in solenoid force and motion that are to be reduced. It will be appreciated, however, that within systems designed to minimize changes in absolute solenoid force and motion, a single temperature sensor may be added to the present system to register ambient temperature, wherein the estimations of the elevation of temperature for each individual solenoid will take into account the actual measured ambient temperature. By way of example, absolute value applications of this nature may include systems such as those found within factory and laboratory automation systems.
In performing the estimation of temperature elevation within the solenoid, a heat value is maintained for each solenoid. The heat value may range between a lower limit that equates to the ambient temperature and an upper limit which equates to a condition of thermal equilibrium. The heat value is periodically decreased to take into account the heat being dissipated by the solenoid. In periods of relative inactivity, the temperature of the solenoid decreases toward the ambient temperature. In response to periods of solenoid activity, the heat value is periodically increased according to the amount of power being applied to the solenoid. By way of example, incrementing and decrementing the heat value may be performed on a periodic basis wherein the heat value can gradually decrease or increase in response to the changing conditions. Alternatively, the heat value can be recalculated when a change of state has occurred or is necessitated.
It should be appreciated that under certain conditions the heat value may decrease during a period of solenoid activity. For example, if the level of power applied to the solenoid is lowered after an extended period of higher power activation, the thermal dissipation level as a result of the elevated temperature of the solenoid may still exceed the applied power, wherein the temperature and corresponding heat value will be correspondingly decreasing.
Estimation methods are subject to a lack of precision and error accumulation within most applications, wherein system performance diverges over time from the desired operation. It should be appreciated, however, that the operation of a solenoid within the present invention is not subject to substantial error-accumulation because a solenoid subject to sufficient activity, or inactivity, will attain a thermal equilibrium state at either ambient temperature, or the high temperature thermal equilibrium point for the solenoid. The present heat accumulation method is therefore substantially self-calibrating under normal operating conditions as solenoid temperature drops to ambient or reaches an upper thermal equilibrium condition.
The generated estimate of relative solenoid heat accumulation may then be applied for compensating relative solenoid drive power so as to normalize the mechanical outputs of the solenoids. The present invention describes a method of altering the solenoid force value to which each solenoid is driven in response to a calculated heat value. By way of example the electrical drive associated with a desired force value is modulated by a gain value computed in relation to the heat value and constants associated with the behavior of a solenoid within the system.
An object of the invention is to provide for the estimation of thermal heating within a solenoid in response to activations thereof.
Another object of the invention is to provide a mechanism for estimating relative solenoid temperature without utilizing a temperature sensor on each solenoid coil.
Another object of the invention is to provide solenoid drive compensation in response to the thermal heating of the solenoid such that solenoid drive fluctuation with respect to solenoid temperature may be normalized.
Another object of the invention is to provide solenoid drive temperature compensation wherein the amount of compensation is urged toward the average to reduce coil-to-coil variation thereof.
Another object of the invention is to provide a method of estimating coil temperature which may be utilized in a variety of electromechanical systems.
Further objects and advantages of the invention will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the invention without placing limitations thereon.